Rice Genomics and Genetics 2024, Vol.15, No.1, 36-47 http://cropscipublisher.com/index.php/rgg 41 Figure 2 Diurnal changes in intercellular CO concentration in rice (Adopted from Wen et al., 2019) Not only that, Rubisco is also a key carbon-fixing enzyme in photosynthesis, and the reduction of its activity may lead to a decrease in the overall photosynthetic rate. This affects the plant's fixation of carbon dioxide, slowing the plant's growth and development. Some plants and microbes may have adapted Rubisco's structure to adapt to high-temperature environments through evolution. Some heat-tolerant plants may have Rubisco isoenzymes that are more stable to high temperatures, thereby slowing down the negative impact of temperature on their activity. The main effect of high temperature stress on Rubisco is the reduction of its activity, which may lead to abnormal photosynthesis and restriction of plant growth. This is an important consideration for plant productivity in agriculture and ecosystems in situations such as global warming. High temperature stress has multiple adverse effects on the rice photosystem, including damage to structure and function, decomposition of photosynthetic substrates, stomatal closure, ROS accumulation, leaf damage, and instability of photosystem proteins. These changes jointly lead to a decrease in photosynthetic efficiency, which is one of the key factors leading to reduced rice yield due to high temperature stress. Therefore, it is crucial to study how to improve the adaptability of rice to photosynthetic systems under high temperatures and to take corresponding management measures. 3.2 Effect of high temperature stress on photosynthetic substrates High temperature stress has many effects on the photosynthetic substrates of rice, directly affecting the efficiency of photosynthesis and the growth of rice. High temperature stress promotes the decomposition of photosynthetic substrates, especially glucose and sucrose. These substrates are products of photosynthesis and are used to supply the plant's energy needs and carbon fixation. However, under high temperature conditions, enzyme activity increases, causing these substrates to be broken down into metabolites more quickly, reducing the carbon source available for growth and development. This has a direct impact on rice growth (Yan et al., 2011). Starch granules in rice leaves store carbon during photosynthesis to feed periods when photosynthesis is inactive. However, under high temperature conditions, the degradation rate of starch granules increases, which reduces the amount of starch accumulation. This results in rice lacking sufficient carbon sources to maintain growth and development at night or when photosynthesis is limited. Under high temperature conditions, the production of ATP and NADPH in the photosynthesis chain is inhibited. This is because high temperatures cause damage to the electron transport chain, reducing the production of ATP and NADPH. These two are the key energy and reducing power required to maintain photosynthesis and carbon fixation, and their reduction further reduces the efficiency of photosynthesis. High temperature stress causes oxidative stress in leaves, impairing the redox balance. This affects the production and utilization of a variety of photosynthetic substrates. In addition, oxidative stress can also lead to oxidative damage, reducing enzyme activity and the stability of photosynthetic substrates. Under high temperatures, rice may adjust the allocation of carbon sources, allocating more carbon to stress response and defense mechanisms and reducing carbon sources for growth and yield. This change may result in growth restrictions because rice is unable to fully utilize carbon sources to support normal growth processes.
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